The present invention relates to exhaust gas remediation and in particular, a method and apparatus for enhancing the reactivity of the material catalysts found within catalytic converters of cars, trucks and power stations.
Plasma aftertreatment of exhaust gases has previously been identified as a possible remediation technique since non-thermal plasma can induce a host of new chemical reactions due to electron excitation thereby causing the production of an abundant amount of radicals and excited state molecules. Plasma aftertreatments rely upon the generation of high local electric fields which directly produce energetic electrons. These energetic electrons can influence the chemistry, even in the collision dominated regime, because they do not lose much energy in elastic collisions due to their small mass, but instead bounce around and transfer most of their energy to molecules; either dissociating, ionizing, or otherwise exciting them. This excitation and radical production can cause extensive changes in reaction rates; in some cases, as much as a hundred thousand-fold increase.
While the efficiency of non-thermal plasma treatments may be limited by any of several factors; two limitations that especially inhibit efficient utilization of non-thermal plasma discharge in connection with the aftertreatment of exhaust gases are low electrical efficiency and an undesirable reaction pathway. See B. M. Penetrante et al., A Comparison of Electrical Discharge Techniques for Nonthermal Plasma Processing of NO in N2, pp. 679-687 in IEEE Transactions in Plasma Science, Vol. 23, No. 4, 1995, the relevant portions of which are incorporated herein by reference.
In the excited state chemistry of non-thermal plasmas, it is desirable to produce as high an electric field as possible. This would at first seem to simply entail applying high enough voltages to a suitably arranged configuration of electrodes. However, for non-thermal plasma densities, there occurs considerable plasma shielding of the applied fields, even in the collision dominated regime and field limits. See J. H. Whealton and R. L. Graves, A Exhaust Remediation Using Non-Thermal (Plasma) Aftertreatments: A Review, @ Proceedings of the 1995 Diesel Engine Emissions Reductions Workshop, Vol. 23, LaJolla, Calif., the relevant portions of which are incorporated herein by reference. This space charge shielding is due to the charge imbalances arising within the plasma because of the higher mobility of electrons as compared to positive ions. Accordingly, the electric fields are generally limited by high voltage breakdowns which in turn lead to a low impedance discharge. Further, plasma shielding effects take place for electric (or electromagnetic) fields at frequencies below the electron plasma frequency. As a result of this shielding, the volume of plasma
As noted above, problems also arise with respect to the chemistry path. While both oxidation and reduction reaction pathways are possible avenues for the dissociation of NOx their respective chemistries differ. The oxidation reaction pathway will result in the production of compounds that include N2O and nitric acid. Since nitric acid is toxic, it""s generation is to be avoided, especially in automobiles, trucks or other mobile applications. On the other hand, the dissociative attachment occurring in the reduction reaction pathway is more favorable since it leads to the formation of benign N2. Unfortunately, the above noted difficulty in reaching high E/N where E is peak electric field (V/M) and N is gas density (molecules/M3), in prior art discharges has prevented development of a plasma aftertreatment device that will achieve a high fraction reduction of NOx as opposed to the less desirable oxidation.
In view of the above problems and deficiencies of plasma aftertreatments of exhaust gases, the present invention was developed.
Accordingly, it is an object of the present invention to provide a method of non-thermal plasma aftertreatment of exhaust gas whereby the electric fields will no longer be limited by high voltage breakdowns and associated low impedance discharge.
It is another object of the present invention to provide a method of non-thermal plasma aftertreatment of exhaust gas whereby significant increases in the arc transition electric field are achieved by incorporation of high frequency power of order 100 MHz and higher.
It is a further object of the present invention to provide a method of non-thermal plasma aftertreatment of exhaust gas whereby any plasma shielding effects taken place for fields at frequencies below the electron plasma frequency are significantly reduced through the use of high frequency power.
It is yet another object of the present invention to provide a method of non-thermal plasma aftertreatment of exhaust gas whereby efficient coupling of high frequency power to produce high electric fields is achieved through application of resonant cavities.
It is an additional object of the invention to provide a method of non-thermal plasma aftertreatment of exhaust gas whereby any surface charging of dielectrics, to the extent it does occur, will enhance the fields in the next (reverse field) half cycle.
It is another object of the present invention to provide a method of non-thermal plasma aftertreatment of exhaust gas whereby the electric fields can be ramped up so quickly, for example, 40 ps for 5 Ghz microwaves, that the probability of the discharge initiating near threshold is substantially reduced thereby resulting in a tilt of the reaction pathway toward the more desirable reduction pathway as opposed to the oxidation pathway.
It is yet another object of the present invention to provide a method of non-thermal plasma aftertreatment of exhaust gas that will enable much higher E/N, and a continuous production of atomic nitrogen by employing bursts of high frequency RF or microwave electric fields having fast risetimes i.e. the risetime of each wave cycle, and many cycles per burst; specifically, high-power microsecond bursts of microwaves having frequencies in the range of about 100 MHz to several tens of GHz to provide pulse risetimes approximately 100 times shorter than that of the prior art whereby thousands of electric field oscillations per burst of microwave power are achieved.
It is an additional object of the invention to provide a method of catalyst performance enhancement, with-and/or without a plasma, whereby the active catalyst particles (i.e. metal crystallites) are exposed to high electric fields and preferentially heated (as compared to the catalyst substrate) by these fields to increase reactivity only very locally in places where the reactions are taking place for purposes of providing controlled and highly localized enhanced catalyst reactivity.
It is further an object of the present invention to provide a method and apparatus of exhaust gas treatment for use with existing smart solid catalytic converters to synergistically improve the reactivity and efficiency of the catalyst and the exhaust remediation via the application of the high-power pulsed microwave fields to the catalyst with and/or without the creation of a plasma.
It is another object of the present invention to provide a new plasma generator for the non-thermal plasma aftertreatment of exhaust gases whereby plasma shielding is reduced and a higher E/N in the bulk of the plasma is achieved, reducing the N2 vibrational excitation risk and permitting high electron temperatures.
It is an additional object of the invention to provide a method of non-thermal plasma aftertreatment of exhaust gas whereby field reduction caused by plasma shielding is prevented, atomic nitrogen is produced and made available during substantially the entire treatment process, any surface charging of the dielectrics that may occur will enhance fields in the next half cycle, field limits due to arc breakdowns are significantly increased and higher fields are achieved.
The present invention is further directed to a method of non-thermal plasma aftertreatment of exhaust gases having reduced plasma electric field shielding, increased availability of atomic nitrogen, exploitation of surface charging of dielectrics, avoidance of (low field) threshold initiated discharges, and achievement of a higher high-energy tail on the electron distribution function.
In summary, the present invention provides a method for non-thermal plasma aftertreatment of exhaust gases the method comprising the steps of providing high frequency, 100s of MHz to 10s of GHz, high power bursts of low-duty factor microwaves sufficient to generate a plasma discharge and passing a gas to be treated through the discharge so as to cause dissociative reduction of the exhaust gases. The choice of microwave frequency will depend not only on optimization for achieving the desired physical and chemical effect, but also on engineering considerations associated with deployment of an exhaust treatment device. These considerations include, but are not limited to, cost and availability of the microwave source and the size of the catalyst region to which the microwaves will be applied.
The present invention is also directed to a waveguide reactor for generating non-thermal plasma for aftertreatment of exhaust gases, the reactor comprising a pulsed microwave source for generating the plasma discharge, a transmission system (i.e. waveguide, coaxial line, etc.) for conveying the microwave power to the flow reactor, a flow reactor which directs the microwave fields to the plasma and/or catalyst region whereby exhaust gases subjected to the generated plasma discharge are caused to be dissociated.
The present invention is further directed to a waveguide reactor as set forth above and operatively associated with a material catalyst whereby the short burst, high-power microwave fields generated by the reactor are caused to increase the reactivity of the catalyst surface, with or without the generation of a plasma, for purposes of exhaust gas remediation.
These and other objects of the present invention will become apparent from the following detailed description.